1. Field of the Invention
This invention relates to the stabilization of pyrolysis gasoline (“pygas”), and more particularly to the first stage hydrogenation of pygas.
2. Description of the Prior Art
Crude oil fractions such as a straight run naphtha from a crude oil still are conventionally steam cracked in an olefins unit to produce light olefins and aromatics, valuable chemicals in their own right. Pygas is a valuable by-product of such steam cracking because it is generally high octane and within the general gasoline boiling range of from about 100 to about 435° F., and can be used as a finished gasoline blending stream after undergoing certain processing before blending.
Because pygas is derived from steam cracking complex hydrocarbon streams such as naphthas, it can carry with it a large amount of widely varying catalyst poisons that interfere with the aforesaid pre-blending processing of pygas. The amount and severity of pygas poisons is unusually severe as compared to other gasoline producing streams, e.g., gasolines from catalytic cracking units. This makes pygas pre-blending processing quite detrimental to catalyst life during such processing.
Also unlike other gasoline streams used for finished gasoline blending, pygas, before first stage hydrotreating contains substantial amounts of gum precursors, and has poor oxidation stability.
Accordingly, pygas is challenging to stabilize and otherwise process before gasoline blending is undertaken.
The first stage of pygas processing before blending is often hydrotreating over a Group VIII metal catalyst (iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum) to selectively hydrogenate gum precursors such as diolefins, acetylenics, styrenics, dicyclopentadiene, and the like while not hydrogenating significant amounts of mono-olefins, aromatics, and other gasoline octane enhancers. Competitive adsorption causes diolefins and acetylenics to be hydrogenated preferentially over mono-olefins and aromatics thus removing gum tendencies while maintaining octane value. Paraffins are left unchanged or mildly isomerized which can help gasoline value.
Sometimes several stages of selective hydrogenation are carried out.
Second stage hydrotreating is often done on a BTX (benzene, toluene, and xylenes) fraction of pygas for removal of sulfur and other impurities.
The poison severity usually found in pygas can severely reduce first stage hydrogenation catalyst activity and catalyst life. For example, while sulfur, carbonyls, basic nitrogen, and gums/coking tend to be temporary catalyst poisons, arsenic, mercury, lead, and phosphorous tend to be more permanent poisons. Other permanent poisons include trace silicon oxide and corrosion metal oxide dusts which tend to plug catalyst pores. Also polysiloxanes thermally decompose and permanently poison palladium or nickel catalysts.
Guard beds can be employed upstream of a first stage hydrotreater to remove such poisons, but this is an expensive approach, and it is not always physically possible or otherwise practical to install guard beds and regeneration capability.
Thus, it is very desirable to have a pygas first stage hydrogenation catalyst that remains robust as to both selective hydrogenation activity and catalyst life when exposed to the pygas poison severity without resorting to a guard bed or other processing to remove or neutralize poisons before such first stage hydrotreating.
Group I B metals (gold, silver, and copper) have heretofore been used as a catalyst for the selective hydrogenation of unsaturates, see U.S. Pat. No. 5,068,477 to Berrbi. Berrbi's patent does not suggest directly or indirectly the use of promoters to make a pygas first stage hydrogenation Group VIII metal catalyst more robust to poisons carried in the pygas.
Group I B metals have also been suggested to be used with Group VIII metal on a support of silicon dioxide to remove alkynes, dienes and mono-olefins from olefin streams for polymerization over a metallocene catalyst or from pyrolysis gases produced in plastic recycling plants, see U.S. Pat. No. 6,204,218 to Flick et al. This patent also does not relate to the rendering of pygas hydrotreating catalyst more robust by the use of promoters.
European Patent No. EP O 738 540 Al to Zisman et at. discloses a method for the selective hydrogenation of acetylene in the gas phase using a catalyst containing alkali metal, chemically bound fluorine, and a support. Zisman et al. disclose that when the atomic ratio of fluorine to alkali metal is in (the range of 1.3:1 to 4:1 the catalyst is more resistant to deactivation to sulfur impurities. Zisman et al. optionally include silver in their catalyst but not as a promoter against sulfur poisons since Zisman et al. achieve their desired protection against sulfur deactivation when silver is not present in their catalyst.
U.S. Pat. No. 4,404,124 to Johnson et al. also discloses a method for the selective hydrogenation of acetylene in the gas phase using a catalyst containing palladium, silver, and alumina. Johnson et al. disclose that a high loading of the catalyst “is expected” to make the catalyst less sensitive to arsenic in the gaseous feed.
Thus, Zisman et al. and Johnson et al. 1) teach the use of silver as an active catalytic part of a catalyst for the selective hydrogenation of gaseous acetylene; 2) lead away from the use of silver as a promoter for sulfur poisons; and 3) only speculate as to the effect of silver high loading in respect of arsenic in the context of an acetylene selective hydrogenation catalyst. Further, neither of these patents suggest directly or indirectly the use of promoters to make a hydrogenation catalyst for normally liquid pygas more robust in the presence of the wide variety of poisons (which include sulfur poisons) normally found in a pygas feed.